Radiometers serve to detect the temperature of an object in a contactless manner by the detection of the infrared (IR) radiation emitted by the object by means of an IR detector. The area of the object, the radiation of which is detected by the detector, is called radiation measuring surface or only measuring surface of the temperature measuring device. In order to measure the temperature reliably, it is important to know the location and the size of the measuring surface. The location and the size of the measuring surface depend on the alignment of the measuring device, the construction of the detector, the properties of an IR optics and the measuring distance. Different kinds of construction of sighting devices for the visualization of measuring surfaces are known, which produce a visible marking inside and/or at the edge of the measuring surface.
The marking can, for example, comprise several light points which are produced at the edge of the measuring surface by means of one laser or several lasers and adapted projection lenses. According to DE 196 54 276 A1 the light points are produced with laser beams extending to each other in a skew manner, each of which are directed into the desired direction by means of a deviating prism.
It is known from EP 0 867 699 A2 and U.S. Pat. No. 5,368,392 to mark the measuring surface by a continuous bordering line. The bordering line may be produced by a rotating laser. According to another embodiment, a laser beam is deflected by a rotating mirror such that it produces a circular bordering line on the object. If the laser beam is moved at a frequency of more than 30 Hz it seems to draw a continuous bordering. Another embodiment of said documents uses a beam splitting device for splitting one laser beam into a plurality and for marking the periphery of the measuring surface with several points. An optical fiber bundle may be used as beam splitting device. Alternatively, also several individual lasers may be employed.
Documents EP 0 458 200 A2 and U.S. Pat. No. 5,172,978 disclose a radiometer whereof the sighting means is coaxially arranged about a combination of a detector and a condensing lens. With said condensing lens the detector is projected on the object in a focused manner. The measuring surface merely has the size of the sensor surface of the detector. The sighting device is formed by at least one concentric Fresnel lens, with which an additional light source is projected onto the object, likewise in a focused manner. The IR beam path is separated from the beam path of the visible light. According to an embodiment described in said documents two concentric Fresnel lenses may be employed. According to another embodiment described in said documents an annular mirror may be employed together with an annular lens for projecting the visible light.
A similar radiometer is known from DE 100 36 720 A1. The annular lens known from EP 0 458 200 A2 is called a toroidal lens. Since said document discloses that marking light hits the rear side of the toroidal lens, here, too, the IR beam path is separated from the beam path of the visible light. According to an embodiment the IR optics finitely projects the detector along an optical axis so that the measured beam path is a hyperboloid with one shell. The corresponding marking beam path is formed in that light from the light source is deviated into straight paths at the toroidal lens, with the paths extending to each other in a skew fashion and to the optical axis on a hyperboloid surface which encloses the measured beam path. The marking beam path therefore shows a narrowest contraction. For producing the marking beam path, the toroidal lens has a non-rotationally symmetrical lens body, the rear side of which is a conical surface and the front side of which is a piece-wise, continuous screw-shaped annular surface. Disclosed are annular surfaces with one, two, ten and thirty-six sections.
It is desirable to provide an improved radiometer, sighting device and corresponding methods.